1. Field of the Invention
This invention relates to telephone technology and more particularly to a fail-safe operation method for maintaining loop stability between the transmit and receive paths of a speaker phone and for switching from a full-duplex into a half-duplex mode when loop instability is detected.
2. Description of the Background Art
Traditional speaker phones function in a half-duplex mode, allowing only one person at a time to speak. When one talker (near-end) is speaking in a half-duplex system, the signals received from the other talker (far-end) are blocked until the near-end speech is either completed or interrupted by a stronger signal. Often, while a talker is speaking, the signals may be blocked if someone at the other end makes moderate noises and activates the microphone. In other cases, talkers are required to shout into the speaker phone in order to be heard by the other end. It is generally very difficult, if not impossible, in a half-duplex system to interrupt current talkers while they are speaking. All of these conditions are very annoying in a teleconferencing situation.
Room acoustic echo has always been one of the most severe problems in hands-free speaker phone systems. Acoustic echoes occur when the far-end speech sent over the telephone line comes out from the near-end loudspeaker, feeds back into a nearby microphone, and then travels back to the originating site. Talkers at the far-end location can hear their own voices coming back slightly after they have just spoken. One method of eliminating these irritating acoustic echoes is to apply an echo suppressor to switch off the microphone while the other end is talking. This results in the half-duplex operation currently implemented on standard speaker phones. More sophisticated acoustic echo cancelers are available for full-duplex operation to improve interactivity in teleconferencing. Acoustic echo cancelers employ adaptive filtering techniques to model the impulse response of the conference room in order to reproduce the echoes from the speaker signal. The estimated echoes are then subtracted from the out-going microphone signals to prevent these echoes from going back to the far-end.